The present invention relates to a system and method for suppressing noise and more particularly to a system and method for suppressing noise produced by high-speed rotors of aircraft through the distribution of sources on the rotor blade and/or by developing a radial force.
Noise generated by aircraft is a problem for both the passengers in the aircraft as well as the community over which the aircraft flies. One source of noise which is often dominant is the noise due to rotors. In a helicopter, there are two such rotors: the main rotor and the tail rotor. In transport aircraft, the source of noise is the propeller.
The noise produced by rotors comes about by virtue of the movement of the blade through the air. Traditionally, the noise sources have been subdivided into three categories: loading, thickness, and quadrupole noise. Loading noise results from the lift and drag forces the rotor exerts on the air. If the forces on the air vary with time, then the noise is called unsteady loading noise. Thickness noise is produced by displacement of the air due to the finite volume of the rotor blade. Quadrupole noise is due to strong gradients in the fluid near the rotor blade surface. Quadrupole noise as used herein is a generic term which includes the noise from shocks, the Reynolds stresses, the boundary layer, and the trailing vortex sheet. For sound to be radiated from a rotor, it is not necessary for the magnitude of these sources to vary with time (in blade-fixed coordinates). The noise itself results from the rapidly changing position of the blade (volume, forces and quadrupoles) with respect to the observer. This effect determines both the time-dependence and amplitude of the noise generated. For subsonic motion, both the amplitude and directionality are proportional to (1-M.sub.r).sup.-n, where M.sub.r is the Mach-number at which the source approaches the observer and n is a positive number. The factor n influences the amplitude at which the blade radiates sound due to its motion.
Several techniques have been used to reduce rotor noise. Most of these techniques have concentrated on changing the source characteristics of the noise. One such method discovered by H. Hubbard (see H. H. Hubbard, "Propeller-noise Charts for Transport Airplanes," N.A.C.A. TN 2968 (1953), and H. H. Hubbard and L. N. Lassiter, "Sound from a Two Blade Propeller at Supersonic Tip Speeds," N.A.C.A. Rep., 1079 (1952)), involves the reduction of the tip speed. The amplitude of radiated noise becomes very large for points on the blade, where M.sub.r.sup..about. 1, and Hubbard's method reduces noise by reducing the maximum value that M.sub.r can attain. M.sub.r is the velocity of the blade divided by the speed of sound in the direction from a point on the blade to the observer. The maximum velocity on the blade is at the tip. Thus, by reducing tip velocity, the maximum possible value of M.sub.r is reduced.
Another method for reducing rotor noise which was first proposed by D. Bliss (see U.S. Pat. No. 3,989,406, Bliss, D. B., "Method of and Apparatus for Preventing Leading Edge Shocks and Shock-Related Noise in Transonic and Supersonic Blades and the Like") involves the sweeping of the blades. Normally, rotor blades are made by stacking the aeordynamic centers of the blade sections along a straight radial line. To sweep a blade means to stack the blade sections along a curved arc. This arc can be located in a plane of rotation called in-plane sweep, or on a helical surface. Initially, this method was used with ducted fan blades, but it has been utilized with unducted rotors as well. The sweep of the blades reduces the shock formed on the blade, and thereby reduces the source strength. At a later date, Hanson discovered that unducted swept blades show reduced noise by virtue of another effect. First, approximately all points on a straight (radial) blade attain their maximum value of M.sub.r at the same retarded time. The term "retarded time" describes the time at which the blade emitted the sound. If the blade is swept, the points attain their maximum value of M.sub.r at different retarded times. Hanson (see D. B. Hanson," The Influence of Propeller Design Parameters on Far Field Harmonic Noise in Forward Flight", 1979 American Institute of Aeronautics and Astronautics Paper 79-0609 and D. B. Hanson, "Study of Subsonic Fan Noise Sources", 1975 American Institute of Aeronautics and Astronautics Paper 75-468 recognized that sweeping the blade is equivalent to shifting the phase of the sound wave emitted by each such point on the blade.
Another method for reducing noise, proposed by Succi, (see "Design of Quiet Efficient Propellers", SAE paper 790584, Business Aircraft Meeting, April, 1979) involves the changing of the number of blades. At any instant, there is only one blade that approaches the observer at its fastest rate, and it is this blade that is the greatest noise source. By reducing the strength of the source, by distributing the loading and thickness over a greater number of blades, noise is reduced.
"Tip mass injection" has been proposed by R. White (see Pegg, R. J., "Insights into the Nature and Control of Rotor Noise," NASA Conference on Aircraft Safety and Operational Problems, NASA SP416) for reducing helicopter rotor noise. This noise is related to the interaction of the main rotor blade with its wake, and is known as blade/vortex interaction noise. White proposed injecting air into the tip vortex to change the character of the tip vortex and thereby change the blade/vortex interaction noise.
Despite all of these attempted solutions, no known system satisfactorily reduces helicopter rotor noise.
It is therefore a principal object of the present invention to provide a system and method for suppressing aircraft rotor noise more effectively than known systems and methods.
Still another object of the present invention is to provide a system and method for suppressing aircraft rotor noise by separately cancelling rotor loading and thickness noise by using mass suction and/or radial forces.